The invention relates to a device for implanting self-expanding endoprostheses. Endoprostheses of the generic type can be inserted, for example, in veins, bile ducts or urinary tracts to maintain patency. They are also used to prevent recurring stenoses in the sense of an elastic recoiling or a cicatrized constriction following balloon dilation of arteries.
In most known devices, the axial length of the endoprosthesis is considerably longer when it is folded up than when it is expanded. Therefore, positioning in vessels, etc., is relatively difficult, since the exact length and location of the endoprosthesis is not apparent until after it unfolds. In most known devices, the endoprostheses can be released by way of a relative motion between the tubular body and the core element.
A common feature of all the devices currently available on the market, however, is the fact that once the endoprosthesis is partially released, it can no longer be folded back up again. This means that once even just a small piece of the endoprosthesis is released, the device can only be removed from the body by completely releasing the endoprosthesis and leaving it in the body. Moreover, once the endoprosthesis is partially released, it can only be pulled proximally but not pushed, since the expanded distal end of the endoprosthesis would inevitably injure the vessel in which it was supposed to be introduced if it were pushed. Also, the shape of the endoprosthesis would be affected by this forcing action; it would be compressed and its supporting elements would buckle.
If it is discovered when the endoprosthesis is released and unfolded that the final location does not correspond to the desired position, the possibility should thus exist of pulling the endoprosthesis back into the device again after it is partially released so that it can accordingly be repositioned in a folded state.
A device of this type, described as an instrument for inserting a self-expanding implant, is outlined in [design patent] DE-GBM G 90 10 130.8. This device consists basically of an outer casing inside of which is an axial hollow core. The core has a diameter that increases incrementally at its distal end, whereby the distal end of the core strikes against the distal end of the casing. Around the distal end of the core is a gripping component that firmly holds a self-expanding implant in such a way that it can be separated. In order to produce a friction-locking connection between the core or the gripping component and the expanding prosthesis, the gripping component is made of a high-friction material. In addition, another type model provides for coating the gripping component with an agent for gluing the expanding implant in such a way that it can be separated.
A disadvantage of this device is the fact that the outer casing must tightly enclose the expanding implant so that a friction-locking connection is ensured between the gripping component and the self-expanding implant as proposed. As a result of this, a relative displacement creates a great deal of friction between the core and the outer casing, and naturally between the expanding implant and the inner wall of the outer casing, so that consequently forces exerted to release the expanding implant and to withdraw it are great. Furthermore, there is a risk that due to the relatively strong forces being exerted, reliability with respect to precise positioning of the implant will be adversely affected accordingly, because in addition to the friction between the expanding implant and the outer casing, there is also the friction between the core and the outer casing along the entire remaining length of the device. Moreover, it is more expensive to ensure by creation of the corresponding friction coefficients that the relative movement actually occurs as intended between the expanding implant and the outer casing, although there the normal force on the friction surfaces is higher by the total expansion force of the expanding implant than the normal force between the core and expanding implant.
With this device it is crucial, in order to achieve a very specific friction between the core and the expanding implant, that a normal force also be required that is just as precise. In this case, the amount of normal force is determined by the elasticity of the outer casing, the elasticity of the expanding implant, and the elasticity of the core, on the one hand, and by the inner diameter of the outer casing, the thickness of the expanding implant and the outer diameter of the core, on the other. Moreover, the expansion force of the expanding implant further determines the amount of normal force; it works against the normal force. The required normal force must be met exactly in order to produce a specific friction force; on the other hand, however, the normal force is very sensitive to the determining factors cited. Mass production of this device therefore presents great problems.
The additional proposal in the aforementioned design patent with respect to coating the gripping component with an agent for gluing the expanding implant in such a way that it can be separated brings with it the risk that the expanding implant, especially when stored for longer periods, will no longer expand on its own after the outer casing is pulled back, since its opening force is too weak, or that the adhesive coat will undergo chemical changes with time and consequently that the desired friction-locking connection will no longer be guaranteed. Applying the adhesive coat also presents problems; every effort must be made to prevent the adhesive coat from getting between elements of the expanding implant, which must move toward one another when the implant expands. Separable adhesive agents derive their properties from the fact that they always retain certain rheological properties; they do not harden. Consequently, the danger exists that adhesive agents that are properly applied originally will begin to run during the time the device is stored and that the expanding implant will become stuck together.
Another type model of the design patent provides for supplying the gripping component with a roughened surface. However, with a roughened surface, there is the danger that the expanding implant will become deformed and, as a result, that its shape will be affected, particularly when it expands.
It should be noted in general that a sure method of operation cannot be achieved in every case by means of type models such as those proposed in the aforementioned design patent, since with the manufacturing of expanding implants and, in particular, with the type of compression, high tolerances with respect to accuracy to gauge and expansion force must be reckoned with.